Integrated circuit fabrication makes use of many different optical processes. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, InP, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices. Fourier filters are sometimes used in the systems that implement these optical processes, such as inspection systems, alignment systems, and exposure systems.
At least two previous methods exist for implementing a Fourier filter, which methods are a mechanical method and a liquid crystal method. The mechanical method utilizes physical rods that are placed so as to block the regular pattern bright areas present at the Fourier plane. The liquid crystal method uses a one-dimensional or two-dimensional, fully blocking or fully transmissive liquid crystal system with a digital output to block the regular pattern bright areas present at the Fourier plane.
The mechanical method has several disadvantages. First, this method induces ringing. Since the transition from full transmission to no transmission occurs instantaneously at an interface between the two extremes, the system has a step input response. The output response contains a large amount of ringing that reduces the signal to noise ratio, especially in the array region. Second, this method can only be implemented as a one-dimensional Fourier filter. Since the rods span the entire Fourier plane from one end to the other, blocking a single spot requires blocking the entire coordinate along the axis of the rod that defines the spot. Third, since the rods must have a relatively large diameter in order to be structurally sound, only a limited number of rods can be used, or else the entire Fourier plane would be blocked. Fourth, this method entails the typical issues associated with mechanical systems, such as reliability issues, large size, and low operating speed.
The liquid crystal method has many of the same problems as the mechanical method. The liquid crystal Fourier filter operates on the principle of light scatter, therefore providing areas of either full transmission or zero transmission. Because of this, ringing is present after the image is filtered. Although the two-dimensional implementation allows for the filtering of individual spots, the one-dimensional implementation presents the same issues as the mechanical method when trying to block individual spots. In addition, since this method utilizes light scatter, it cannot block light only partially, thus it can behave only as a digital output spatial light modulator. Light scatter also tends to introduce wave front phase aberrations.
What is needed, therefore, is a system whereby problems such as those described above can be overcome, at least in part.